Herbicidally active carboxylic acids such as benzoic acids, phenoxy carboxylic acids, pyridine carboxylic acids and quinoline carboxylic acids have been found to be useful in controlling the growth of weeds in various crops. In particular, these carboxylic acids are classified as growth regulating herbicides and act as synthetic auxins in preventing the growth of weeds in crops. In use to control the growth of unwanted plants, a herbicidally effective amount of one or more of these carboxylic acid herbicides is applied to the locus of weeds, generally with one or more agriculturally acceptable carriers, surfactants and other additives.
It is known to apply herbicidally active carboxylic acids to susceptible vegetation in their acid forms. However, their herbicidal effectiveness is poor because the water-insoluble acid forms do not penetrate the leaves of susceptible vegetation sufficiently for fast and efficient control and eradication. Therefore, the herbicidally active carboxylic acids are traditionally converted from the acid forms into esters or salts, in order to enhance water solubility and leaf penetration properties. Both the ester and salt forms of such herbicidally active carboxylic acids are available commercially.
A conventional method of preparing carboxylic acid salts is to react an aqueous carboxylic acid solution with an alkali metal- or ammonium-containing alkaline solution to produce the corresponding carboxylic acid salt and water. The carboxylic acid salt is typically then dried to form a solid herbicide.
U.S. Pat. No. 3,013,054 discloses the preparation of sodium 2-methoxy-3,6-dichlorobenzoate. In the process disclosed, 2-methyoxy-3,6-dichlorobenzoic acid (116 g; 0.5 mol) is dissolved in 500 cc of methanol and treated with a solution of sodium hydroxide (20 g; 0.5 mol) in 100 cc of methanol. Methanol is removed by distillation in vacuo over a steam bath and the solid residue is slurried with 100 cc of cold dry ether, filtered, pressed dry, and dried completely in a vacuum oven to give the desired salt, sodium 2-methoxy-3,6-dichlorobenzoate.
U.S. Pat. No. 5,266,553 suggests to prepare solid salts by neutralization of the corresponding aryloxycarboxylic acids with a base, followed by conversion of the resulting aqueous solution into the solid salts by evaporating water under controlled conditions. The solid salts can also be prepared by evaporating water from the corresponding commercially available aryloxy salt solutions.
One issue arising with these known methods is cost. In particular, to dry the carboxylic acid salt after its formation, the salt solution must be heated. As a result of the large volume of water used in the preparation of the acid solution and produced during formation of the carboxylic acid salt, it is expensive to provide the necessary amount of heat energy to dry the carboxylic acid salt solution.
Another issue related to the production of carboxylic acid salts is the pH of the carboxylic acid salt solutions. Various methods have been employed to control the pH of the carboxylic acid salt solutions and the resulting carboxylic acid salts. In particular, as the resulting carboxylic acid salts are generally re-dissolved in water for their eventual use, it is important to have salts that have a suitable pH when dissolved in water. One method of controlling the pH of the carboxylic acid salts is to neutralize the solution as needed prior to drying. For example, buffers, such as alkaline compounds, have been combined with the carboxylic acid salts to produce the desired pH for the carboxylic acid solution.
U.S. Pat. No. 5,266,553 describes a method of producing carboxylic acid salts by forming the carboxylic acid salts from the carboxylic acid and an alkaline compound, followed by reacting the carboxylic acid salt with a second alkaline buffer compound to produce a carboxylic acid salt having increased water solubility. An alternative method of controlling the pH of the salts is to prepare different batches of carboxylic acid salts and mix the batches to provide the desired pH value. However, it is difficult to dry salts, particularly when they are sticky, and produce a consistent product.
Another issue with the known methods is production yield and purity. The prior art provides processes, which are applicable for use on a laboratory scale.
However, for production on an industrial scale, the problems of low yield and purity remain unsolved.
Accordingly, there is a need for an improved process for preparing solid, free-flowing and readily water-soluble carboxylic acid salts, in particular applicable for use on an industrial scale.